I. Field of the Invention
This invention relates generally to an implantable cardiac stimulating device and more particularly relates to a cardiac stimulating device capable of using multiple electrodes for automatic capture and threshold verification. Each of several electrodes are utilized for sensing, pacing and capture verification within an electrically continuous area of the cardiac muscle. During predetermined periods, the stimulation device verifies the effectiveness of a stimulation impulse by applying the stimulation impulse to the heart muscle via one electrode and then the electrical signal resulting from the induced cardiac muscle activity is evaluated by one or more of the other electrodes.
II. Discussion of the Related Art
Cardiac stimulators typically include a pulse generator, limited power supply, electrical leads, and an integrated circuit or microprocessor based controller. In order to maximize use of the limited power supply, it is desirable to set the lowest output energy that reliably causes depolarization of the corresponding cardiac muscle resulting from an electrical stimulus generated by the pulse generator. To ensure the reliability of pacing, it is common practice to determine the minimum output energy that induces a cardiac depolarization ("the energy threshold") manually during patient follow-ups, and then set the pacemaker's output at this minimum setting plus a wide error margin, usually double or triple the minimum effective energy. This error margin is meant to account for the changes in energy requirement that may occur over the time between the patient follow-ups. It is far more economic if the pacemaker can track the changes of the minimum required energy, and adjust its output energy settings to that, with a much smaller error margin. In order to do so, it is necessary that the pacemaker is able to verify if an electrical stimulus is effective. This automatic verification is known as auto capture.
Over the years single or dual chamber cardiac pacers have evolved, whereby capture verification and threshold are automatically determined. The dual chamber cardiac pacers may be programmed such that sensing occurs in one chamber of the heart and pacing is directed to another chamber of the heart. The sensing amplifiers of such devices generally have a refractory period of sufficient length to mask the initial responses of the heart to the stimulation pulses or stimulated heartbeats. This refractory period is necessary to block out artifacts caused by polarization of the electrodes coupled to the lead which act as both pacing and sensing electrodes.
Mulier, in U.S. Pat. No. 3,757,792 describes a pacemaker coupled to two leads each having an electrode. One of the electrodes is designated for normal pacing and sensing and the other electrode is dedicated to sensing of heartbeats that are induced by the other electrode, wherein both electrodes are situated on the ventricles. Each electrode of the Mulier device is limited to a specific task, one for stimulating and the other for detecting. The present invention recognizes the advantages to including multiple pacing electrodes, wherein the capture of each electrode's stimulus may be verified by the other electrode(s). Hence, electrodes capable of functioning both for stimulation and detection are desirable.
Other cardiac pacing devices have been described that verify the effectiveness of a stimulus from one electrode using the same electrode for verification. When using a single electrode for verifying the effectiveness of its own stimulus, various techniques are required to minimize detection of the polarization built up on the pacing electrode or alternatively, the device must use T-wave secondary heart beat characteristics. Representative of such devices are, for example, those disclosed by Bowers, U.S. Pat. No. 3,920,024; Jirak, U.S. Pat. No. 3,949,758; Auerbach et al., U.S. Pat. No. 4,055,189; Lewyn et al., U.S. Pat. No. 4,114,627; Rickards, U.S. Pat. No. 4,228,803; Wittkampf et al., U.S. Pat. No. 4,305,396; Decote, Jr. U.S. Pat. No. 4,708,142; and Callaghan et al., U.S. Pat. Nos. 4,955,376 and 4,969,460.
Greeninger in U.S. Pat. No. 5,324,310 describes use of both atrial and ventricular electrodes to determine a global inter-cardiac signal which thereby helps a physician verify capture manually. The Greeninger device requires a DDD pacer and two bipolar leads, wherein one lead is positioned in the atrium and the other lead is positioned in the ventricle. A physician then evaluates the global signal to determine whether capture has occurred. Markowitz in U.S. Pat. No. 5,601,615 describes a pacing device capable of verification of atrial capture by pacing in the atrium and verifying depolarization utilizing an electrode positioned in the ventricle. In order to determine ventricular capture, the '615 device paces the ventricle and then after responsive atrial activity, verifies that no wave passes an electrode positioned in the ventricle. Further, verification of capture in a single chamber pacing mode of the '615 device occurs by applying an early pacing stimulus and verifying the absence of depolarization where it would be expected after a non-disturbed cycle. The '615 device does not utilize more than one electrode in the same electrically continuous area (for example, the ventricular muscle mass or the atrial muscle mass) to verify capture of one of the electrode's stimulus. Hence, there is a need for a positive type of confirmation of capture, wherein the device is able to function in either the atria or the ventricles independently and does not require the presence of electrodes in both the atria and ventricles or conduction through the AV node between the atria and ventricles. The present invention addresses these and other needs that will become apparent from a review of the disclosure herein.